Dormancy in perennial plants is controlled by phytochrome, a protein hormone. In its active form, produced by red light (660nm), bud dormancy is prevented and height growth continues. Phytochrome slowly degrades to an inactive form in the dark. In the absence of sufficient active phytochrome, buds are set and height growth ceases.
In order to insure that seedlings reach a desired commercial size on schedule, it is often important to provide supplemental light at night to prevent premature dormancy. The intensity need not be great; 40 foot candles is enough. Furthermore, since red light inhibits stem elongation, it is best if the light is intermittent. As little as 1 part in 30 is enough as long as no dark period is longer than 30 minutes.
Since only red light is important and other wavelengths are much less efficient or even detrimental (far red (730 nm) reverses the effect of red light), light sources vary tremendously in their efficiency. Tungsten incandescent bulbs are the least efficient but can be turned on and off rapidly and repeatedly without loss of bulb life. They are commonly used in greenhouses as a fixed over-head array in spite of the large number of fixtures required and the day time shading of natural sunlight they impose on the plants.
Fluorescent lights are much more efficient light sources, but cannot be turned on and off repeatedly without greatly shortening bulb life. Fixed arrays of fluorescent lights are more expensive than incandescent, and their bulk creates more shade. They are practical only if mounted on a moving boom, such as many greenhouses have for watering. The lights remain on most of the night and the boom moves back and forth from one end of the greenhouse to the other which creates the intermittent light. However, there are two major disadvantages: (1) providing electric power to a moving water boom requires a complex mechanical system with a correspondingly large potential for safety and reliability problems; and (2) because it operates at night, it is usually unattended. If it fails, the problem may not be discovered until the next day.
The most efficient light source is the high pressure sodium arc light. Its spectrum is concentrated very close to the most effective wavelengths for dormancy prevention and it has little or no far red. However, the fixtures are very expensive, and it is impossible to turn the lights on and off rapidly. Furthermore, the light is very intense, more so than needed, so much of their output is frequently wasted. However, only one such light would serve if it were centrally mounted overhead, and if its light could be spread evenly over the plants in the greenhouse.